Conventionally, a frequency converter used for converting, e.g., an FM reception signal of an FM receiver into an intermediate-frequency signal, is arranged as shown in FIGS. 1 to 3.
In a circuit in FIG. 1, reference numeral 31 denotes a frequency mixer for mixing an FM reception signal (high-frequency signal) and a local oscillation signal. The mixer 31 is formed in a bipolar integrated circuit, 30. Reference numeral 32 denotes an intermediate-frequency transformer for extracting an intermediate-frequency component of 10.7 MHz from an output of the frequency mixer 31. The transformer 32 is connected between a Vcc power source terminal 34 and a frequency mixing output terminal 35 outside the integrated circuit 35. Reference numeral 33 denotes a ceramic filter (center frequency is 10.7 MHz), for obtaining desired selectivity, connected to the secondary coil of the intermediate-frequency transformer 32.
The frequency mixer 31 comprises a first npn transistor Q11 the base of which receives an FM reception signal (high-frequency signal) input, a second npn transistor Q12 the base of which receives a DC bias potential, a resistor R connected between the emitters of the npn transistors Q11 and Q12, a first current source Ia connected between the emitter of the first npn transistor Q11 and a ground potential GND, a second current source Ib connected between the emitter of the npn transistor Q12 and the GND, a third npn transistor Q13 the collector-emitter path of which is connected between the Vcc power source terminal 34 and the first npn transistor Q11, a fourth npn transistor Q14 the collector-emitter path of which is connected between the Vcc power source terminal 34 and the second npn transistor Q12, a fifth npn transistor Q15 the emitter of which is connected to the emitter of the third npn transistor Q13, the base of which is connected to the base of the fourth npn transistor Q14, and the collector of which is connected to the frequency mixing output terminal 35, and a sixth npn transistor Q16 the emitter of which is connected to the emitter of the fourth npn transistor 14, the base of which is connected to the base of the third npn transistor Q13, and the collector of which is connected to the frequency mixing output terminal 35. Local oscillation signal inputs are applied between the bases of the third and fifth npn transistors Q13 and Q15 and between the bases of the sixth and fourth npn transistors 16 and 14.
In the circuit in FIG. 1, in order to match the secondary coil of the intermediate-frequency transformer 32 with the impedance of the ceramic filter 33, a turn ratio of the secondary coil to the primary coil of the intermediate-frequency transformer 32 must be decreased. Therefore, a large intermediate-frequency conversion gain is difficult to obtain.
On the other hand, in a circuit shown in FIG. 2, a frequency mixer 41, an impedance converting emitter follower 42 connected to the output of the frequency mixer 41, and a resistor 43 one terminal of which is connected to the output of the emitter follower 42 are arranged in a bipolar integrated circuit 40. Reference numeral 44 denotes a Vcc power source terminal; 45, a first frequency mixing output terminal connected to the output terminal of the frequency mixing circuit 41; and 46, a second frequency mixing output terminal connected to the other terminal of the resistor 43. Reference numeral 47 denotes an intermediate-frequency tuner consisting of an LC tuner for tuning an intermediate-frequency component of 10.7 MHz of the output from the frequency mixer 41. The tuner 47 is connected between the Vcc power source terminal 44 and the first frequency mixing output terminal 45 outside the integrated circuit 40. Reference numeral 48 denotes a ceramic filter, for obtaining desired selectivity, connected to the second frequency mixing output terminal 46 outside the integrated circuit 40.
In the circuit in FIG. 2, since impedance matching between an output impedance of the frequency mixing circuit 41 and the ceramic filter 48 is performed by the emitter follower 42, a large intermediate-frequency conversion gain can be obtained. However, since the emitter follower 42 has no signal amplitude limiting function, an intermediate-frequency signal at a level higher than a desired level is supplied to an intermediate-frequency amplifier (not shown) connected to the output of the ceramic filter 48.
In a the circuit shown in FIG. 3, a frequency mixer 51, a resistor 52 connected to the load and output of the frequency mixer 51, and an intermediate-frequency limiter 53 are arranged in a bipolar integrated circuit 50. Reference numeral 54 denotes a Vcc power source terminal, and reference numeral 55 denotes a frequency mixing output terminal connected to the output terminal of the intermediate-frequency limiter 53. Reference numeral 56 denotes a ceramic filter, for obtaining desired selectivity, connected to the frequency mixing output terminal 55 outside the integrated circuit 50.
In the circuit in FIG. 3, a signal amplitude limiting function of the intermediate-frequency limiter 53 can prevent supply of an intermediate-frequency signal having a level higher than a necessary level to the sequential intermediate-frequency amplifier. However, since an intermediate-frequency transformer or an intermediate-frequency tuner is not connected to the load of the frequency mixer 51, an unnecessary frequency component except for an intermediate-frequency signal generated by the frequency mixer 51 cannot be suppressed, thereby degrading spurious features of the circuit.
As described above, in a conventional frequency converter, when an intermediate signal is extracted from an output of a frequency mixer through the intermediate-frequency transformer, a large intermediate-frequency conversion gain is difficult to obtain. When the intermediate-frequency signal is extracted from the output of the frequency mixer through an emitter follower, an intermediate-frequency signal having a level higher than a necessary level is supplied to the intermediate-frequency amplifier connected to the following stage. As a result, when the intermediate-frequency signal is extracted from the output of the frequency mixer through an intermediate-frequency limiter, spurious features of the circuit are degraded.
The present invention has been made to solve the above problem, and has as its object to provide a frequency converter in which a large intermediate-frequency conversion gain can be obtained, supply of an intermediate-frequency signal having a level higher than a necessary level to a sequential intermediate-frequency amplifier can be prevented, a signal of an unnecessary frequency component except for an intermediate-frequency signal generated by the frequency mixer can be sufficiently suppressed, and which is free from the degradation of spurious features.
Disclosure of Invention
According to the present invention, there is provided a frequency converter comprising a frequency mixer formed on a semiconductor integrated circuit and an impedance conversion/signal amplitude limit differential amplifier connected to an output of the frequency mixer, wherein an output terminal of the frequency mixer and an output of the differential amplifier are connected to first and second frequency mixing output terminals serving as external terminals of the semiconductor integrated circuit, respectively.
With the above arrangement, since impedance matching between an output impedance of the frequency mixer and that of a intermediate-frequency filter can be performed by the impedance conversion/signal amplitude limit differential circuit, a large intermediate-frequency conversion gain can be obtained, and supplying of an intermediate-frequency signal having a level higher than a necessary level to a sequential intermediate-frequency amplifier can be prevented.
In addition, since an intermediate-frequency tuner, is connected to the load of the frequency mixer, the unnecessary frequency component signal, except for an intermediate-frequency signal generated by the frequency mixer, can be sufficiently suppressed, and spurious features of the circuit are not degraded.